1. Field of the Invention
The present invention relates to a switch circuit. More particularly, the present invention relates to a switch circuit to control on/off of a high voltage source.
2. Description of Related Art
FIG. 1 shows a circuit diagram of a traditional switch circuit to control on/off of a high voltage source, in which a voltage VD is required to provide a supply voltage to a load 10. The voltage VD is supplied from a high voltage source VIN through a transistor 11. A drain terminal and a source terminal of the transistor 11 are coupled to the high voltage source VIN and the load 10 respectively. A resistor 15 is coupled from the drain terminal of the transistor 11 to a gate terminal of the transistor 11. The resistor 15 is used to provide a bias voltage to turn on the transistor 11. A drain terminal and a source terminal of a transistor 12 are coupled to the gate terminal of the transistor 11 and a ground respectively. A gate terminal of the transistor 12 is coupled to an output terminal of an inverter 14. An input terminal of the inverter 14 receives a control signal SN. The transistor 12 is controlled by the control signal SN through the inverter 14. Therefore, the transistor 12 can be turned on to switch off the transistor 11 in response to the disabling state of the control signal SN. However, when the transistor 12 is turned on, the resistor 15 will consume a power PR, it is given by,
                              P          R                =                              V            IN            2                                R            15                                              (        1        )            wherein R15 is a resistance of the resistor 15.
The high voltage source VIN would be as high as 350VDC when a high line voltage is connected. Therefore, a significant power loss will be produced at the resistor 15. The resistor 15 in high resistance such as several mega ohms can be used to reduce the power loss. However such the resistor 15 in high resistance is not appropriate to be built into an integrated circuit. Therefore, it is desirable to develop a high efficiency switch circuit to switch on/off of the high voltage source, specifically to develop an integrated circuit to switch on/off of the high voltage source.